Are common garden chemicals a health risk?

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Disclosure statement

Ian Musgrave receives funding from the National Health and Medical Research Council for screening herbal medicines for toxins. He has previously held ARC and NHMRC funding for work related to Alzheimer's disease.

As the weather warms and days lengthen, your attention may be turning to that forgotten patch of your backyard. This week we’ve asked our experts to share the science behind gardening. So grab a trowel and your green thumbs, and dig in.

Gardening is good for your health, but it can pose some risks if you’re not careful. For example, you should use sensible protection against the sun to prevent cancer, a significant cause of death in Australia.

Gardening in Australia also requires, to varying degrees depending where in the country you are, pesticides, herbicides and fertilisers. There is an enormous number of agents with multiple formulations, depending on what you are doing, what plants you are tending, the size of your garden and the kind of soil.

These garden treatments are designed to be specific and potent, so they can be applied less often and work (mostly) only on the things you want them to work on. All of them – even the “natural” ones such as sulfur dusts to control caterpillars and mildew – are chemicals, which means they have health risks.

Regulation of chemicals

While not perfect, gardening product safety is regulated in Australia. Let me introduce you to the Australian Veterinary Medicines and Pesticides Authority. This body regulates pesticide (a substance that kills pests such as insects and weeds) and herbicide (a substance that kills only weeds) products sold in Australia.

The AVPMA regularly reviews products for safety concerns, though the reviews may be decades apart. It co-ordinates with World Health Organisation bodies and its counterparts in Europe, Canada and the United States.

Given the sheer number of compounds and formulations available, I can’t possibly cover the safety of all chemicals, or even all groups of chemicals. If you are concerned about a particular product, you can search the AVPMA site for the ingredients in a given pesticide, herbicide or fertiliser.

This may be tricky, as some will not necessarily have a chemical name on them, just the trade name. However, most pesticides and herbicides from reputable companies should have a material safety data sheet (MSDS) with them. This should give you the details you need to check through the AVPMA site.

There are several ways to deal with pests: repel, smother or poison them.from shutterstock.com

Pesticides

There are three ways to deal with pests (well, four if you count laboriously picking them off your plants): repel, smother or poison them. All these can harm people if they are exposed to significant quantities. But in an urban garden, exposure to pesticides is typically small and of limited duration.

Common smothering pesticides are oils such as petroleum oils used to control, say, leaf pests on citrus, or pests in a variety of other circumstances. If you apply these often, without gloves, you might get skin irritation; or lung irritation if you breath the spray in. So always follow the directions, which include wearing gloves and spraying so the wind doesn’t blow the spray back into your face.

Pyrethrums are found naturally in some chrysanthemum flowers.from shutterstock.com

Modern poisoning insecticides include the pyrethrums which are found naturally in some chrysanthemum flowers. Both the natural pyrethrum and synthetic pyrethroids have low toxicity to humans – particularly at the doses found in garden products. Continuous use of pyrethroid insecticides has no health implications for humans if instructions are followed.

Neonicotinoids are synthetic insecticides that mimic nicotine, which is toxic to insects. These have a place in pest control if used thoughtfully and sparingly. Unlike pyrethroids, these insecticides target a pathway in the insect nervous system shared with humans, so could potentially harm us.

Chronic use of neonicotinoids in a human gardening population has not been assessed for long-term health effects, but a small study of agricultural workers has shown no effect of chronic exposure. Another small study, however, suggests some association with memory loss.

Herbicides

Again, there is a bewildering variety of herbicides, depending on what weedy species is being targeted and how the weed is being killed.

However, the IARC’s determination was based on only a small number of animal studies and didn’t include a number of animal studies where glyphosate did not cause cancer. It also said nothing about risk; that is, what is the likelihood glyphosate would cause cancer at the concentrations humans are usually exposed to?

The European Food Safety Authority and the AVPMA have evaluated the evidence and determined that under appropriate handling conditions applicable to general backyard gardeners, there is no risk to humans.

As a comparison, a homemade herbicide of salt, vinegar and soap that is claimed as a replacement for glyphosate is more toxic than glyphosate.

Another herbicide for woody weeds, like blackberry, is Triclopyr. This can cause eye and skin irritation, but has no serious long-term health impacts if proper safety procedures are followed.

Fertilisers

There are many formulations and varieties of fertilisers depending on soil type and location (where I live is basically sand). Health risks are basically related to long-term inhalation of fine particles, which could cause breathing difficulties. Once again follow the safety instruction.

Caveat

Just because a given product is not, or minimally, toxic to humans, that does not mean you should apply it to your garden by the bucket load. Always apply any garden chemical with care and thought, using the right amount at the right time for the right purpose.

Any agent you apply or spray can cause adverse reactions if you don’t use it as directed. Getting “organic” garlic and soap insecticide spray in your eyes will hurt like billy-o, just like the latest you-beaut synthetic pyrethrum spray, even though both are pretty much non-toxic to humans.